Venus is characterised today as the planet from hell yet may have been closer to a cherished idea that it was once Earth's twin, a conference heard on Wednesday.

Data sent back by a European scoutship suggests that even though Venus' surface is super-heated and smothered by a lethal atmosphere, the planet may once have been lush with water.

The second rock from the Sun is similar in size and age to Earth, a resemblance which unleashed sci-fi speculation last century that it could provide us with a home away from home.

Those dreams were crushed when space probes in the 1960s found the planet's surface hot enough to melt lead, with no sign of any water except as traces in an atmosphere of carbon dioxide and sulphuric acid.

Those traces, scanned in detail by instruments aboard the European Space Agency (ESA) craft Venus Express, are relics of what may have been large quantities of water that were lost to space or stripped away by the Sun, investigators say.

Observations presented on Wednesday at the European Planetary Science Congress in Potsdam, Germany, reveal intriguing differences in water vapour in the Venusian atmosphere at altitudes ranging from 10 kilometers (6.2 miles) up to 110 kms (68 miles), which is far above the clouds.

The ratio of "heavy water," which contains the isotope deuterium, to normal water is nearly twice as high in the upper levels compared to the lower lower atmosphere.

"Water vapour is a very rare species in the Venusian atmosphere: if it were in liquid form now, it would cover the surface of Venus with just a few centimetres (inches) of water," said researcher Emmanuel Marcq of the LATMOS laboratory in France in a press release.

"However, we believe Venus once had large volumes of water that have since escaped into space or been stripped away by the solar wind."

The data sent back by Venus Express demonstrate that the heavier water containing deuterium has not been able to flee Venus's gravity as easily as normal H2O, said Marcq.

"This enrichment of heavy water provides strong evidence that water loss is occurring in the upper atmosphere and that Venus was probably more humid and Earth-like in the distant past."

Other investigations into Venus' past conclude that the planet was hit by runaway global warming in which water vapour -- a heat-trapping greenhouse gas -- played a key role.